This invention relates to a key assigner for use in an electronic musical instrument which generates musical tones in response to voltages representing the notes of depressed keys and is generally called a musical synthesizer, and, more particularly, to a key assigner with an improved response characteristic of analog circuits for sampling and holding note voltages.
The electronic musical instrument to which the novel key assigner of this invention is applicable is of the type disclosed in my copending patent application Ser. No. 601,945, filed Aug. 4, 1975, in which independent note voltage generating circuits and key gate circuit are provided, a note voltage, (i.e., a voltage representing a note and determining an oscillator frequency) being sampled by a key gate circuit corresponding to each key, the sampled note voltage being further sampled by channel gate circuit respectively corresponding to a plurality of channels equal in number to the maximum number of tones to be generated simultaneously, the sampled note voltage being held by charging capacitors provided for respective channels and the note voltage corresponding to a depressed key being sampled by controlling the key gate circuits in synchronism with the control of the channel gate circuits and in accordance with a scanning operation for detecting the ON-OFF states of the key switches, whereby a plurality of tones are simultaneously generated by voltage controlled oscillators in accordance with the note voltage held by the capacitors.
For the purpose of controlling a plurality of key gate circuits in accordance with the depressed keys, it is necessary to scan all switches and detect the ON-OFF states of the respective keys for producing gate control signals. However, a high speed scanning is generally necessary to scan and detect a plurality of key switches in time sharing without affording a feeling of unnaturalness to the audience. On the other hand, in order to sample the note voltage in synchronization with the gate control and to charge respective capacitors with the sampled voltages, it is necessary to decrease the sampling speed by taking into consideration the charge voltage response characteristic determined by the resistance time constant of the analog circuits including the capacitors and the gate circuits.
According to the invention described in said copending patent application, for the purpose of decreasing the sampling speed, respective key codes are statically held by temporarily latching the key codes in respective channels which are delivered at a high synchronous rate, and the key codes of respective channels thus latched are multiplexed in time-sharing at a low synchronous rate by the operation of a plurality of multiplexing circuits and the multiplexed codes are decoded to produce gate control signals corresponding to the key codes. However, according to that procedure, it is necessary to latch and statically hold the key codes of respective channels and, accordingly, latching circuits of the same number as the channels must be provided. This makes the circuit construction complicated and expensive. Moreover, this method is defective in that the musical tone frequency fluctuates in the rise portion of the tone due to the adverse effect of transient voltages produced at the time of charging capacitors.